Cold rolling is an important stage in the production of long products in the metallurgy industry. Its objective is to reduce the thickness of the product input. The sheet metal products are usually destined for the motor vehicle and foodstuffs industries.
The rolling thus consists of reducing the thickness of a metal strip by means of plastic deformation. For this purpose, the strip circulates continuously between two rotating rolls, known as work rolls, with parallel axes, which delimit between one another a gripping space which is commonly known as an air gap, and to which force is applied. The reduction of thickness of the strip is then obtained by compression. This device constitutes a stand of a rolling mill. The use of a plurality of stands in succession into which the strip passes simultaneously constitutes a rolling mill tandem.
The work rolls are rotated at a regular speed. As it passes into the stands of the rolling mill, the speed of the strip increases, taking into account the decrease in its thickness and the maintaining its width.
For metallurgical reasons, the variations of thickness at the output from the tandem must be as slight as possible. For this purpose, different regulation loops are used.
Thus, it is common to continuously measure the linear speed of the strip output from the first stand, the thickness of the strip input into, and output from the first stand, and the thickness output from the final stand.
For example, it is known to correct the thickness by acting on the air gap of the work rolls of the first stand according to the thickness measured at the input of the first stand. The air gap is the distance which separates the two work rolls.
Similarly, it is known to modify the air gap of the work rolls of the first stand according to the thickness measured at the output from this first stand.
It is also known to modify the speed of rotation of the rolls of the first stand according to the thickness of the strip output from the first stand.
Finally, it is known to adjust the speed of rotation of the rolls of the final stand on the basis of the thickness measured at the output from this final stand.
These correction methods permit reduction of the variations of thickness of the strip, but remain insufficient to take into account complex phenomena which occur in a rolling mill.
In addition, in the particular context of hot rolling, a method is known from document EP-A1-0 000 454 for compensation for the effects of variation of slippage on the traction between stands, so as to maintain this traction at a constant value in order to maintain the width of the rolled product. This method is based on the principle of maintaining the speed of the strip at the two ends between stands.
Within the context of cold rolling, the physical phenomena involved are different. Thus, the traction between stands does not have any effect on the width of the rolled product. Consequently the problem of maintaining the traction between stands at a constant value solved by the method described in document EP-A1-0 000 454 is not important within the context of cold rolling. In addition, the matter of controlling the traction of the strip in a cold rolling installation is easily resolved by regulating traction using tractiometers. These devices are generally not used during hot rolling of a metal sheet, since they are very difficult to implement.
It is also usual, in cold rolling mills, to allow the traction between stands to increase naturally when the rolling speed decreases. Contrary to the hot rolling methods (where the traction is kept constant) it is this variation of traction between stands which gives rise to variation of slippage at the output from the stand upstream.